One of a number of measuring methods for ascertaining fill level in a container is the travel time, measuring method. In the travel time, measuring method, for example, microwaves, or radar waves, are transmitted via an antenna apparatus, and echo waves reflected on the surface of the medium are received back after the distance dependent, travel time of the measuring signal. From half the travel time, the fill level of the medium in a container can be calculated. The echo curve represents, in such case, the received signal amplitude as a function of time, wherein each measured value of the echo curve corresponds to the amplitude of an echo signal reflected on a surface at a certain distance. The travel time, measuring method is essentially divided into two methods of ascertainment. Time difference measurement is a first method of ascertainment, in which the time required for a broadband, wave, signal pulse to move over a traveled path is ascertained. A further, widely employed method of ascertainment is that wherein the sweep frequency difference between a transmitted, frequency modulated, high frequency signal and the reflected, received, frequency modulated, high frequency signal is ascertained (FMCW—Frequency Modulated Continuous Wave). In the following, there is no limitation to a particular method of ascertainment.
Long used in process measurements technology are group radiator antennas, such as planar antennas or antenna arrays, horn antennas and parabolic antennas. Planar antennas are characterized by compact construction and cost effective manufacture, as compared with other antennas. Conventionally, planar antennas are designed as free field antennas, which usually radiate and receive linearly, or also circularly, polarized waves. Some embodiments of such planar antennas are described, for example, in DE 101 18 009 A1. As printed antenna structures, they differ, in turn, on the basis of their primary HF radiating element in terms of resonant structures, such as e.g. patch, slit, monopole and dipole antennas, and non resonant slit antennas, such as e.g. tapered slit antennas and Vivaldi antennas, as well as combinations of these resonant and non resonant structures. The feeding, or exciting, of the radiating elements occurs, normally, via strip lines (microstrip lines). Other line structures for the feeding of the elements, such as coplanar, and slit, lines, are likewise possible. Through simple photolithographic manufacture as printed circuits, such planar antennas are very suitable for mass production.
Another group of printed antennas, or exciting structures, are those, which produce a certain EM field distribution. As known from EP 1083413 B1, TE01 mode is produced with planar, slit radiators. This mode has for fill level measurements in bypasses and sounding tubes, as hollow conductors, the advantage, that the TE01 mode of the high frequency, measuring signal has, on the basis of its field distribution, very low attenuation and propagates almost uninfluenced by container wall disturbances, such as welded seams and holes. An option is also to work with the fundamental mode TE11 in a round, hollow conductor. A special property of the TE11 mode is that its travel velocity propagates best in the hollow conductor, in comparison with the other, higher modes.
Especially presenting problems in process measurements technology is condensation and accretion of process media on the antenna, as a result of the temperature gradient in the process tank. Thus, condensate causes major attenuation of the high frequency, measuring signal, and, moreover, the radiation characteristic, or measuring properties, of the antenna is/are altered. In the case of fill level measurements in the sounding tube, or hollow conductor, higher modes of the high frequency, measuring signal are excited, which propagate with different group travel velocities in the hollow conductor. Due to the interference of the higher modes of the high frequency, measuring signal with the wanted signal, accuracy of measurement of the system is degraded.
Fundamentally, planar antennas are disadvantaged by the disturbance sensitivity of the measuring on the basis of condensate, in comparison to horn, and parabolic, antennas, since the radiation direction of the wave is usually perpendicular to the plane, in which the antenna is oriented. For this reason, the surface of the planar antenna is orthogonal to the gravitational field of the earth, or parallel to the surface of the fill substance of the medium. The volatile components of the fill substance to be measured condense on the cold surface of the planar antenna, whereby drops form, which then only drop off after reaching a certain size, when the surface tension is no longer sufficient to hold the drops. Since the condensation of the evaporating fill substance, or medium, on the planar radiating surface of the planar antenna cannot be prevented, it is attempted to improve, via a structural measure, the dropping off, and shedding, behavior of the condensate on the planar antenna. Such an embodiment of an adapted planar antenna is described in U.S. Pat. No. 6,684,697 B1, wherein the planar antenna is inclined at an angle to horizontal H. Through the inclined position, the force of gravity has also a force component parallel to the surface of the planar antenna, whereby the condensate, driven by this additional force component (normal force), runs together to form larger drops and, at a given position, drops off. The changing of the wavefront, or the radiation direction of the radiation lobe, by the inclined orientation of the planar antenna, is compensated by a different phase control of the rows of the antenna element. Furthermore, in U.S. Pat. No. 6,629,458 B1, an embodiment of a planar antenna is disclosed, in which a filled, hollow cone is placed in front of the planar antenna as antenna protection element, or a radome with a fill material of a dielectric, thermally insulating material is emplaced, which has the same effect, such as earlier described, that the condensate can drop off of the surface of the antenna. Such antenna protection elements in the form of membranes, lenses or cones, are also applied in the case of horn antennas and parabolic antennas for protecting the reflection surfaces of the antennas from condensate formation or deposits of medium.
Disadvantageous in the case of such antenna protection elements of the state of the art is that the antenna protection element, for preventing attenuation of the high frequency, measuring signal, can only be embodied with smallest height and slope possible. Due to the small slopes of the structures, the antenna protection element sheds the condensate only poorly. Furthermore, the antenna protection elements of the state of the art excite higher disturbance modes, whereby the measuring performance of the measuring device is lessened.